Feedforward loop to stabilize current-mode switching converters

ABSTRACT

A circuit includes a current sensor to sense a switching current flowing at input side of a switching DC-DC converter. An output capacitor filters an output voltage at an output side of the switching DC-DC converter. A feed-forward circuit passes a portion of the sensed switching current to a feedback path on the output side of the switching DC-DC converter simulating a changing effective series resistance (ESR) of the output capacitor to facilitate operating stability in the switching DC-DC converter.

TECHNICAL FIELD

This disclosure relates to switching power supplies, and moreparticularly to a circuit that adjusts the effects of an equivalentseries resistance (ESR) of an output filter for a switching converter tofacilitate stabilization of the converter.

BACKGROUND

A switching regulator (or converter) is a circuit that uses a powerswitch, an inductor, and a diode to transfer energy from input tooutput. At the output, a storage capacitor receives the energy generatedin the inductor via switching of the power switch. The basic componentsof the switching converter can be rearranged to form a step-down (buck)converter, a step-up (boost) converter, or an inverter (flyback), forexample. Feedback and control circuitry can be nested around thesecircuits to regulate the energy transfer from the inductor to thestorage capacitor to maintain a constant output within normal operatingconditions.

The most common control method for controlling the switching converteris via pulse-width modulation (PWM). This method takes a sample of theoutput voltage and subtracts this from a reference voltage to establisha small error signal (VERROR). This error signal is compared to anoscillator ramp signal. A comparator outputs a digital output (PWM)signal that operates the power switch. When the circuit output voltagechanges, VERROR also changes and thus causes the comparator threshold tochange. Consequently, the output pulse width (PWM) also changes. Thisduty cycle change then moves the output voltage to reduce the errorsignal to zero, thus completing the control loop.

One issue with PWM converters is related to operating stability of thecontrol loops that maintain the output voltage in regulation. Thecontrol loop for the converter has a gain factor that varies over theoperating frequency of the converter. The gain factor is influenced byPoles and Zero's that are derived from the transfer function of thecircuit and are influenced by the different components of the converter.For example, the output storage capacitor in the converter has aparameter referred to as equivalent series resistance (ESR) which isderived from the frequency in which the storage capacitor receivesenergy from the input side of the converter. The ESR of the storagecapacitor contributes a Zero in the control transfer function whichcontributes to the overall stability of the control loop (e.g., cancelsa dominant Pole in the loop). Over time and temperature, however, theESR of the storage capacitor can change which changes the placement ofthe Zero in the frequency domain for the control transfer function andthus can destabilize the control loop.

SUMMARY

This disclosure relates to a circuit that adjusts the effects of anequivalent series resistance (ESR) of an output filter for a switchingconverter to facilitate stabilization of the converter. In one example,a circuit includes a current sensor to sense a switching current flowingat input side of a switching DC-DC converter. An output capacitorfilters an output voltage at an output side of the switching DC-DCconverter. The feed-forward circuit passes a portion of the sensedswitching current to a feedback path on the output side of the switchingDC-DC converter simulating a changing effective series resistance (ESR)of the output capacitor to facilitate operating stability in theswitching DC-DC converter.

In another example, a circuit includes a current sensor to sense aswitching current flowing at input side of a switching DC-DC converter.An output capacitor filters an output voltage at an output side of theswitching DC-DC converter. The feed-forward circuit passes a portion ofthe sensed switching current to a feedback path on the output side ofthe switching DC-DC converter. The feed-forward circuit changes afrequency response of a control transfer function affected by theeffective series resistance (ESR) of the output capacitor to stabilizethe DC-DC converter. A high pass filter in the feed-forward circuitfilters DC currents of the sensed current and passes AC ripple currentsof the sensed current to the feedback path to change the frequencyresponse of the control transfer function.

In yet another example, an integrated circuit includes a switchingcircuit to switch a current in an inductor at the input side of theswitching DC-DC converter. A current sensor senses the current flowingin the inductor. A ratio circuit selects a ratio of the current sensedby the current sensor. An output capacitor filters an output voltage atan output side of the switching DC-DC converter. A feed-forward circuitpasses the portion of the selected current from the ratio circuit to afeedback path on the output side of the switching DC-DC converter. Thefeed-forward circuit alters a location of a control transfer functionZERO defined by the effective series resistance (ESR) of the outputcapacitor. A high pass filter in the feed-forward circuit filters DCcurrents of the sensed current and passes AC ripple currents of thesensed current to the feedback path to alter the location of the controltransfer function ZERO defined by the ESR to stabilize the switchingDC-DC converter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a circuit that adjusts the effects ofan equivalent series resistance (ESR) of an output filter for aswitching converter to facilitate stabilization of the converter.

FIG. 2 illustrates an example diagram depicting movement of poles basedon the effects equivalent series resistance (ESR) of an output filterfor a switching converter.

FIG. 3 illustrates an example circuit that employs a high pass filter ina feed-forward path to adjust the effects of an equivalent seriesresistance (ESR) of an output filter for a switching converter tofacilitate stabilization of the converter.

FIG. 4 illustrates an example circuit that employs a high pass filterand a ratio circuit in a feed-forward path to adjust the effects of anequivalent series resistance (ESR) of an output filter for a switchingconverter to facilitate stabilization of the converter.

FIG. 5 illustrates an example of switching converter types that can beemployed with a circuit that adjusts the effects of an equivalent seriesresistance (ESR) of an output filter for a switching converter tofacilitate stabilization of the converter.

DETAILED DESCRIPTION

This disclosure relates to a circuit that adjusts the effects of anequivalent series resistance (ESR) of an output filter for a switchingconverter to facilitate stabilization of the converter. In one example,a switching converter can include various types of current-mode DC-DCconverters that can include step-up converters, step-down converters,inverters, and isolation type converters that provide substantially thesame output voltage as supplied by the input voltage. The switchingconverter includes a switching circuit to switch a current in aninductor at the input side of the switching DC-DC converter. A currentsensor senses the current flowing in the inductor. In one example, aratio circuit (e.g., N:1 step-down) selects a portion of the currentflowing in the inductor. As the current is switched in the inductor,rectifiers (e.g., half-wave, full-wave, synchronous rectifiers) on theoutput side of the converter convert the switched inductor current intoa DC output voltage on the output side of the converter. An outputcapacitor filters the output voltage at the output side of theconverter.

Depending on the type, temperature and time that the output capacitor isinstalled, for example, the effective series resistance (ESR) of theoutput capacitor can change which can destabilize the converter (e.g.,move a cancelling POLE in the converter transfer function). Thus, afeed-forward circuit is provided that has the effect of mitigating theeffects of the changing ESR which allows the converter to operate in astable manner. The feed-forward circuit passes the portion of theselected current from the ratio circuit to a feedback path on the outputside of the switching converter. The feedback path enables regulation ofan output voltage for the switching converter. The feed-forward circuitsimulates an increased ESR of the output capacitor to facilitateoperating stability in the converter. A high pass filter in thefeed-forward circuit filters DC currents of the sensed current andpasses AC ripple currents of the sensed current to the feedback path tosimulate the increased ESR.

FIG. 1 illustrates an example of a circuit 100 that adjusts the effectsof an equivalent series resistance (ESR) of an output filter for aswitching converter to facilitate stabilization of the converter. Asused herein, the term circuit can include a collection of active and/orpassive elements that perform a circuit function such as a switchingcircuit or rectifier circuit, for example. The term circuit can alsoinclude an integrated circuit where all the circuit elements arefabricated on a common substrate, for example. The circuit 100 can beconfigured as a switching DC-DC converter (also referred to as switchingconverter). At an input side of the circuit 100, a voltage VIN drives aswitching circuit 110 (e.g., pulse width modulated (PWM) circuit) thatswitches VIN to an inductor 120. Stored energy in the inductor 120 istransferred to an output side of the circuit 100 to be rectified by arectifier circuit 130. Output from the rectifier circuit 130 is filteredby a bulk storage output capacitor 140 to filter a regulated outputvoltage VOUT. The output voltage VOUT is sampled via a feedback circuit150 and fed back to the switching circuit 110 to control the duty cycleof the switching circuit to maintain VOUT at a desired regulationvoltage. The switching circuit 110 can include a comparator (not shown)and a reference voltage to set the regulation of VOUT.

A current sensor 160 senses the current flowing in the inductor 120. Inone example, a ratio circuit (See e.g., FIG. 4) in the sensor circuit(e.g., N:1 step-down, N:1 step-down followed by M:1 step down) selects aportion of the current flowing in the inductor 120. Depending on thetype, temperature, and time that the output capacitor 140 is installed,for example, the effective series resistance (ESR) of the outputcapacitor can change which can destabilize the circuit 100. In additionto ESR variations due to temperature and time, ESR variations can alsobe attributed to the type of capacitor used. For instance, a ceramiccapacitor typically exhibits low ESR values (e.g., in the 10-mOhmrange), while aluminum capacitors have higher ESR values (e.g., in the1-Ohm range). A feed-forward circuit 170 is provided that has the effectof mitigating the effects of the changing ESR in the output capacitor140 which allows the circuit 100 to operate in a stable manner. Thefeed-forward circuit 170 passes the portion of the selected current fromthe current sensor 160 to the feedback circuit 150 on the output side ofthe circuit 100.

The feed-forward circuit 170 simulates an increased ESR of the outputcapacitor 140 to facilitate operating stability in the converter.Essentially, high frequency AC ripple currents generated by the inductor120 and sensed by the current sensor 160, are passed to the feedbackcircuit 150 which has the effect of simulating an increased ESR in theoutput capacitor 140. A high pass filter (not shown) in the feed-forwardcircuit 170 filters DC currents of the sensed current and passes ACripple currents of the sensed current to the feedback circuit 150 tosimulate the increased ESR. The inductor ripple current that isfed-forward to the feedback circuit 150 via the feed-forward circuit 170has the effect of appearing to increase the ripple current at VOUT yetwithout affecting the regulated value of the DC voltage VOUT. The ripplecurrent generally only appears in the feedback circuit 150 however andhas the effect of feeding back a higher frequency to the switchingcircuit 110 which moves an operating ZERO of the circuit 100 in responseto changing ESR.

In one specific example, the output capacitor 140 can be a 330 μFcapacitor (or capacitance if multiple capacitors employed for the outputcapacitor) with ESR varying from 0.05 to 1.6Ω, for example. The ESR ZEROtherefore can move substantially, by orders of magnitude, and thus,affecting stabilization of the circuit 100. For instance, at higher ESRvalues, the ZERO provided by the ESR helps in cancelling non-dominantpoles, but at lower values it moves away to higher frequencies and theeffect of those poles becomes more pronounced—thus leading to looptransfer function instability. The circuits described herein amplify theeffect of the ESR with a feed-forward current loop to facilitate thatthe ESR ZERO does not vary over such a wide range. Thus, feed-forwardinductor current information is directly applied to the feedback circuit150 to simulate an increased ESR and move the ZERO to promote stabilityin the control loop. In this manner, the effect of the capacitor ESR isaugmented by mimicking its ripple voltage with feed-forward inductorcurrent. As a result, the ESR ZERO can be set at a desired frequency. Tomitigate DC errors, the feed-forward circuit 170 can employ a high-passfilter to decrease its gain near DC levels.

FIG. 2 illustrates an example diagram depicting movement of poles basedon the effects equivalent series resistance (ESR) of an output filterfor a switching converter. A bode plot 210 depicts a loop transferfunction with loop gain on the vertical access versus frequency on thehorizontal access. A POLE P1 causes a frequency roll-off of the gain. AZERO (0) appears and has the effect of cancelling a POLE P2 which altersthe slope of the roll-off. The ZERO in this example is supplied by anoutput capacitor having a higher ESR which contributes to loop stabilityby cancelling POLE P2. A second bode plot 220 illustrates a gaintransfer function where the ESR ZERO has moved to a higher frequencythan that depicted at 210. In this example, both POLES P1 and P2 causean increased slope for the roll-off which can contribute to decreasedloop stability. The ZERO which has moved in the example of 200 to thehigher frequency is the result of ESR of the output capacitordecreasing. Thus, the circuits described herein have the effect ofmoving the ZERO depicted at 220 to a lower-frequency region depicted at210 by simulating the effects of increased ESR.

FIG. 3 illustrates an example circuit 300 that employs a high passfilter in a feed-forward path 308 to adjust the effects of an equivalentseries resistance (ESR) of an output filter for a switching converter tofacilitate stabilization of the converter. The circuit 300 includes aswitching circuit 310 to drive an inductor 314 via resistor 318. Theswitching circuit 310 includes a transistor device 320 to switch currentin the inductor 314 and is driven by a driver 324. The driver 324 isdriven by PWM logic 326 which is in turn driven by comparator 328. Aclock 330 drives a ramp generator 334 which supplies a ramp signal toone input of the comparator 328 to control voltage regulation of thecircuit 300. A resistor 336 can optionally feed inductor current tocontrol operations of the ramp generator 334.

Output from the switching circuit 310 is fed though a rectifier 340 togenerate VOUT. The voltage VOUT is filtered by an output capacitor 344which has an equivalent series resistance (ESR) depicted by resistor348. The voltage VOUT is fed back via feedback resistors RFB1 and RFB2through resistor 350 to an integrator 354 having a feedback capacitor356. The integrator 354 utilizes a reference VREF to control VOUT to adesired DC voltage by providing feedback to the comparator 328. Asshown, a current sense 360 samples inductor current and feeds thecurrent to the feed-forward path 308 through a high pass filter 364. Thecurrent sensor 360 can be an inductive loop sensor, for example, or canbe provided via active electronic components such as depicted in FIG. 4described below. Output from the high pass filter 364 is inserted intothe voltage feedback loop via resistor RFB3. The current sensor 360 canprovide a ratio of the inductor current that is fed-forward to thefeedback loop via RFB3. By controlling the ratio (amount of current fedback) and/or the scaling of RFB3, the frequency of the control loop canbe adjusted by moving the ZERO associated with the ESR 348 of outputcapacitor 344. Such adjustments are described in more detail below withrespect to FIG. 4.

FIG. 4 illustrates an example circuit 400 that employs a high passfilter and a ratio circuit in a feed-forward path to adjust the effectsof an equivalent series resistance (ESR) of an output filter for aswitching converter to facilitate stabilization of the converter. Thecircuit 400 includes a switching circuit 410 that drives a transistordevice 414 which in turn drives inductor 420. Output from the inductor420 is rectified via rectifier 424 which supplies VOUT that is filteredby output capacitor 428. Output from VOUT is fed back via resistors RFB1and RFB2 to the switching circuit 410 to control DC regulation of VOUT.

A current sensor circuit 430 samples a ratio of the amount of current inthe inductor 420 and shown as an N:1 ratio, where N represents a smalleramount of the inductor current which is represented as 1. The currentsensor 430 includes a scaling transistor 434 which provides a smalleramount of current than that switched by transistor 414 in the inductor420. Transistor 434 drives an amplifier 438 (e.g., operationalamplifier) which supplies the scaled current from transistor 434 to acurrent mirror (e.g., PMOS current mirror) 440. The current mirror 440can provide an additional M:1 scaling of the scaled current fromtransistor 434, where M represents a smaller amount of output currentfrom the current mirror 440 that from what is supplied at its input. Thecurrent mirror 440 generates two parallel scaled outputs that drivetransistor pair 444 and 448. The transistor pair are coupled at theirrespective gate nodes via a low pass filter that includes resistor 450and capacitor 454. The low pass filter has the effect of sinking DCcurrents from the current mirror 440 (shown as ILPF) while divertinghigh pass ripple currents (shown as IHPF) to the feedback loop via RFB3.Thus, a low pass filter in this configuration has the effect of creatinga high pass filter to supply high pass ripple currents to the feedbackloop which simulate an increased ESR in output capacitor 428 aspreviously described.

Movement of the ESR ZERO can be accomplished via several mechanisms inthe circuit 400. In one example, scaling in the current sensor 430and/or the current mirror 440 can cause more or less inductor current tobe fed back which has the effect of moving the frequency at which theESR ZERO appears in the loop transfer function. In another example,scaling of RFB3 can be utilized to move the ESR ZERO. For example, RFB1and the combination of RFB2/RFB3 set the regulation of voltage VOUT. Ifa 2:1 ratio were employed for example, RFB1 could be set at twice theratio of RFB2 and RFB3. If RFB1 were set to 2 k ohms for example, thenthe combination of RFB2 and RFB3 should equal 1 k ohms, for example.Thus, the ESR ZERO can be adjusted by controlling the ratio of RFB2 toRFB3 while maintaining their collective resistance at 1 k ohms. Forexample, if RFB2 were set at 900 ohms, RFB3 would be set at 100 ohms tomaintain the 1 k ohm collective resistance. If more ESR adjustment weredesired, RFB2 could be lowered while increasing RFB3 yet maintaining thecollective resistance to the desired value (e.g., 1 k ohm in thisspecific example).

FIG. 5 illustrates an example of switching converter types that can beemployed with a circuit 500 that adjusts the effects of an equivalentseries resistance (ESR) of an output filter for a switching converter tofacilitate stabilization of the converter. The circuit 500 can beconfigured as a switching DC-DC converter (also referred to as switchingconverter). At an input side of the circuit 500, a voltage VIN drives aswitching circuit 510 (e.g., pulse width modulated (PWM) circuit) thatswitches VIN to an inductor 520. Stored energy in the inductor 520 istransferred to an output side of the circuit 500 to be rectified byexample rectifier circuits 530, which are described below. Output fromthe rectifier circuit 530 is filtered by a bulk storage output capacitor540 to filter a regulated output voltage VOUT. The output voltage VOUTis sampled via a feedback circuit 550 and fed back to the switchingcircuit 510 to control the duty cycle of the switching circuit tomaintain VOUT at a desired regulation voltage. The switching circuit 510can include a comparator (not shown) and a reference voltage to set theregulation of VOUT.

A current sensor 560 senses the current flowing in the inductor 520. Afeed-forward circuit 570 is provided that has the effect of mitigatingthe effects of the changing ESR in the output capacitor 540 which allowsthe circuit 500 to operate in a stable manner. The feed-forward circuit570 passes the portion of the selected current from the current sensor560 to the feedback circuit 550 on the output side of the circuit 500.As previously described, the feed-forward circuit 570 simulates anincreased ESR of the output capacitor 540 to facilitate operatingstability in the converter. The example rectifier circuits 530 caninclude various forms of rectification. In one example, a half waverectifier 534 (e.g., single diode) can be employed. In another example,the rectifier circuits 530 can include full wave rectification 536(e.g., bridge rectifier). In yet another example, the rectifier circuits530 can include synchronous rectification 538 (e.g., synchronousswitches timed by a control circuit to control rectification via timingof input side switching and output side switching). Substantially anyform of rectification can be employed by the circuit 530.

What have been described above are examples. It is, of course, notpossible to describe every conceivable combination of components ormethodologies, but one of ordinary skill in the art will recognize thatmany further combinations and permutations are possible. Accordingly,the disclosure is intended to embrace all such alterations,modifications, and variations that fall within the scope of thisapplication, including the appended claims. As used herein, the term“includes” means includes but not limited to, the term “including” meansincluding but not limited to. The term “based on” means based at leastin part on. Additionally, where the disclosure or claims recite “a,”“an,” “a first,” or “another” element, or the equivalent thereof, itshould be interpreted to include one or more than one such element,neither requiring nor excluding two or more such elements.

What is claimed is:
 1. A circuit comprising: a current sensor to sense aswitching current flowing at input side of a switching DC-DC converter;an output capacitor to filter an output voltage at an output side of theswitching DC-DC converter; a feed-forward circuit that passes a portionof the sensed switching current to a feedback path on the output side ofthe switching DC-DC converter simulating a changing effective seriesresistance (ESR) of the output capacitor to facilitate operatingstability in the switching DC-DC converter.
 2. The circuit of claim 1,further comprising a high pass filter in the feed-forward circuit tofilter DC currents of the sensed current and to pass AC ripple currentsof the sensed current to the feedback path to simulate the changing ESR.3. The circuit of claim 2, further comprising a ratio circuit to scaleswitching current sensed by the current sensor to a ratio of N:1,wherein N represents a smaller amount of sensed output current than thesensed switching current.
 4. The circuit of claim 3, wherein the ratiocircuit further comprises a scaling transistor that scales a smalleramount of current from a transistor switch device that drives aninductor to generate the switching current.
 5. The circuit of claim 4,further comprising a current mirror to further scale the current fromthe scaling transistor at a ratio of M:1, wherein M represents a smalleramount of sensed output current than scaled by the scaling transistor.6. The circuit of claim 5, wherein the simulated ESR is increased ordecreased by the scaling provided by the scaling transistor or thecurrent mirror.
 7. The circuit of claim 5, further comprising atransistor pair that is driven by the current mirror, the transistorpair are coupled via a low pass filter at their respective gate nodes,the transistor pair and low pass filter acting as a high pass filter todivert high pass ripple currents from the current mirror to the feedbackpath.
 8. The circuit of claim 1, the feedback path further comprises afirst feedback resistor, a second feedback resistor, and a thirdfeedback resistor to sample the output voltage, wherein the combinationof the second and third feedback resistor form a voltage divider withthe first feedback resistor to control the output voltage.
 9. Thecircuit of claim 8, wherein the third feedback resistor is employed toincrease or decrease the simulated ESR by increasing or decreasing itsvalue with the collective value formed by the combination of the secondfeedback resistor and the third feedback resistor.
 10. The circuit ofclaim 1, wherein the circuit is configured as a step-up converter, astep-down converter, an inverter, or an isolation converter.
 11. Thecircuit of claim 1, further comprising a rectifier circuit that includesa half wave rectifier, a full wave rectifier, or a synchronousrectifier.
 12. A circuit comprising: a current sensor to sense aswitching current flowing at input side of a switching DC-DC converter;an output capacitor to filter an output voltage at an output side of theswitching DC-DC converter; a feed-forward circuit that passes a portionof the sensed switching current to a feedback path on the output side ofthe switching DC-DC converter, the feed-forward circuit changes afrequency response of a control transfer function affected by theeffective series resistance (ESR) of the output capacitor to stabilizethe DC-DC converter; and a high pass filter in the feed-forward circuitto filter DC currents of the sensed current and to pass AC ripplecurrents of the sensed current to the feedback path to change thefrequency response of the control transfer function.
 13. The circuit ofclaim 12, further comprising a ratio circuit to scale switching currentsensed by the current sensor to a ratio of N:1, wherein N represents asmaller amount of sensed output current than the sensed switchingcurrent.
 14. The circuit of claim 13, wherein the ratio circuit furthercomprises a scaling transistor that scales a smaller amount of currentfrom a transistor switch device that drives an inductor to generate theswitching current.
 15. The circuit of claim 14, further comprising acurrent mirror to further scale the current from the scaling transistorat a ratio of M:1, wherein M represents a smaller amount of sensedoutput current than scaled by the scaling transistor.
 16. The circuit ofclaim 15, wherein the simulated ESR is increased or decreased by thescaling the provided by the scaling transistor or the current mirror.17. The circuit of claim 12, the feedback path further comprises a firstfeedback resistor, a second feedback resistor, and a third feedbackresistor to sample the output voltage, wherein the combination of thesecond and third feedback resistor form a voltage divider with the firstfeedback resistor to control the output voltage.
 18. The circuit ofclaim 17, wherein the third feedback resistor is employed to increase ordecrease the simulated ESR by increasing or decreasing its value withthe collective value formed by the combination of the second feedbackresistor and the third feedback resistor.
 19. An integrated circuitcomprising: a switching circuit to switch a current in an inductor atthe input side of the switching DC-DC converter; a current sensor tosense the current flowing in the inductor; a ratio circuit to select aratio of the current sensed by the current sensor; an output capacitorto filter an output voltage at an output side of the switching DC-DCconverter; a feed-forward circuit that passes the portion of theselected current from the ratio circuit to a feedback path on the outputside of the switching DC-DC converter, the feed-forward circuit alters alocation of a control transfer function ZERO defined by the effectiveseries resistance (ESR) of the output capacitor; and a high pass filterin the feed-forward circuit to filter DC currents of the sensed currentand to pass AC ripple currents of the sensed current to the feedbackpath to alter the location of the control transfer function ZERO definedby the ESR to stabilize the switching DC-DC converter.
 20. Theintegrated circuit of claim 19, wherein the simulated ESR is adjustedvia a resistor in the feedback path or via the ratio circuit byselecting the ratio of the current sensed by the current sensor.